In the U.S. Ser. No. 545,413 application, a method and apparatus are disclosed for measuring attenuation (dB) across a splice between first and second fibers having ends separated by a distance D.sub.1 by (1) injecting light into the first fiber and measuring the power P.sub.2 in the second fiber when ends of the fibers are separated a distance D.sub.2 and are in substantial axial alignment; (2) again injecting light into the first fiber and measuring the power P.sub.1 in the second fiber when the ends of the fibers are separated by the distance D.sub.1 and in substantial axial alignment, the distance D.sub.2 being much larger than the distance D.sub.1 ; and (3) using the algorithm ##EQU1## where R is a predetermined constant determined under zero attenuation loss conditions. Specifically, R equals P.sub.1 '/P.sub.2 ' where P.sub.1 ' is the power in the light-receiving second fiber when no attenuation loss occurs between the ends of the first and second fibers, and P.sub.2 ' is the power in the same second fiber when the ends of the first and second sample fibers are separated by a distance D.sub.2 '. Preferably, R is determined in the laboratory, with P.sub.1 ' preferably being obtained by simply measuring the power in a sample piece of unbroken fiber (since the fiber is not broken, zero attenuation loss occurs across an "imaginary splice"), with P.sub.2 ' being obtained by subsequently breaking the sample piece of fiber and separating ends thereof by the distance D.sub.2 '. As explained in the U.S. Ser. No. 545,413 application, D.sub.2 should be substantially equal to D.sub.2 ' for equation 1 to directly apply. However, if D.sub.2 is not set equal to D.sub.2 ', appropriate empirical corrections can be made by referring to a chart at page 310 of Optical Fibers for Transmission, John E. Midwinter, John Wiley & Sons Inc., 1979, for single-mode fibers and the chart at page 175 of Fundamentals of Optical Fiber Communications, 2nd ed., Michael Barnoski, Academic Press, 1981, for multimode fibers, the disclosures of these articles being incorporated herein by reference.
In addition, it is preferable to calculate all optical powers P.sub.1, P.sub.2, P.sub.1 ', P.sub.2 ' using the same type of light launch and light withdrawal means. If different means are utilized further empirical correction factors are required. According to a preferred embodiment of the U.S. Ser. No. 545,413 application, light launch and light withdrawal is accomplished directly through a side of the fibers through their buffers in a manner similar to that disclosed in copending U.S. application Ser. Nos. 437,053 and 614,884, the disclosures of which are incorporated herein by reference.